The present invention relates to a manufacturing method and a manufacturing apparatus of an optical fiber coupler used for an optical fiber telecommunication system, an optical fiber sensor, optical measurement or the like. To be further detailed, the present invention relates to a manufacturing method of an optical fiber coupler having an optical characteriztic which is less dependent on the wavelength, and relates to a manufacturing method for an optical fiber coupler which fabricates an optical fiber having a reproducibility far better than the conventional one, and further relates to a manufacturing apparatus of an optical fiber coupler which extends at least one optical fiber in advance and thereafter fabricates the optical fiber coupler by fusion-bonding and extending these optical fibers.
Conventionally, optical fiber couplers have been used as a passive device which simply divides an inputted optical signal to a plurality of outputs ports, or in reverse, couples optical signals inputted from a plurality of ports into one optical fiber. Particularly, the optical fibers are attracting attention from the points of low loss, a good matching property with the optical fiber being an optical transmission path, easy fabrication and the like.
FIG. 11 shows a conventional low-loss optical fiber coupler. This is fabricated in a manner that a plurality of optical fibers 401 are held in parallel, and parts thereof are heated and fusion-bonded, and are further extended until a desired coupling ratio is obtained. FIG. 12 shows a wavelength characteristic of the coupling ratio of this optical fiber coupler. The abscissa represents the wavelength and the ordinate represents the coupling ratio respectively, and the graph shows that a difference in wavelength causes a difference in coupling ratio. The coupling ratio is 50% at 1.3 .mu.m in wavelength, and when the wavelength becomes larger than 1.3 .mu.m, the coupling ratio also becomes larger than 50%, and when the wavelength becomes smaller than 1.3 .mu.m, the coupling ratio also becomes smaller than 50%. This conventional example has a large dependency of the coupling ratio on the wavelength, and therefore the application thereof is limited for example, it is not suitable for such as the wavelength division multiplex tele-communication system using a plurality of wavelengths.
Also, in fabricating a multi-coupling optical star coupler to improve the close touch and the distribution characteristic, a method is often used which fabricates the coupler while twisting optical fibers in fusion-bonding and extending.
FIG. 13 shows a second example of the conventional optical fiber coupler. This was publicly announced by D. B. Mortimore under the title of Wave-length-Flattened Fused Couplers in Electronics Letters Vol. 21, No. 17, pp742, 1985. This is fabricated in a manner that one of two optical fibers 402 is heated and extended in advance, and the optical fibers having outer diameters differing a little are twisted and closely touched together, and are heated, fusion-bonded and extended. Thereby, the dependency of the coupling ratio on the wavelength can be reduced. FIG. 14 shows an example of the dependency of the coupling ratio of this type on the wavelength. The coupling ratio is 50% at 1.3 .mu.m and 1.5 .mu.m; the coupling ratio is a little larger than 50% between 1.3 .mu.m and 1.5 .mu.m; the coupling ratio gradually varies at 1.3 .mu.m and 1.5 .mu.m. And thereby it can be easily understood that the variation in the coupling ratio is small in comparison with that of the wavelength. Such an optical fiber coupler can be used in a wide wavelength band, and therefore it is also called a wide-band optical fiber coupler.
As an example of the wide-band optical fiber coupler, as disclosed in the U.S. Pat. No. 4,798,436, the method of reducing the dependency on wavelength by providing differences among the propagation constants of a plurality of optical fibers is well known. Also, there is a method of fusion-bonding and extending two optical fibers having different refractive indexes of core as described in the U.S. Pat. No. 4,822,126, but this method spoils the matching property of the optical fiber being a transmission path.
In the above-mentioned circumstances, as disclosed in the U.S. Pat. No. 4,798,438, generally there is an approach wherein one of two optical fibers is extended to have a different diameter smaller than the diameter of the other optical fiber in advance, and thereafter it is arranged in a closely touched fashion with the other optical fiber, and fusion-bonded, extended together; and thereby a difference is provided between the propagation constants of the both optical fibers. This fabricating method is called a pre-extension method because one of optical fibers is extended in advance.
However, the optical fiber coupler fabricated by the pre-extension method not only necessitates another process of pre-extension, complicates the manufacturing process, and requires a large-scale apparatus, but also takes much manufacturing time and cost, and therefore this method cannot provide an economical wide-band optical fiber coupler.
An object of the present invention is to provide a manufacturing method capable of continuously performing the pre-extension process and the fusion-bonding/extending process which have been conventionally performed separately, and provide a manufacturing method of fabricating an optical fiber coupler with a far better reproducibility than the conventional one, and further provide a manufacturing method of an optical fiber coupler which fabricates the coupler in a manner that at least one optical fiber is extended in advance, and thereafter these fibers are fusion-bonded and extended.
To solve the above-mentioned problem, the present invention has been achieved in a manner that, in a manufacturing method of an optical fiber coupler wherein a coupling region is formed by fusion-bonding and extending parts of a plurality of optical fibers arranged in a closely touched fashion, at least one optical fiber among a plurality of the above-mentioned optical fibers arranged in a closely touched fashion is extended at a speed different from that of the other optical fibers using a low-temperature heat source softening the above-mentioned optical fibers, and the temperature of the above-mentioned low-temperature heat source is raised to form a high-temperature heat source, and then the extended parts of a plurality of the above-mentioned optical fibers arranged in a closely touched fashion are fusion-bonded, and further the temperature of the above-mentioned high-temperature heat source again, and then the above-mentioned fusion-bonded part is extended.
The use of the manufacturing method of the present invention makes it possible to continuously perform pre-extension and fusion-bonding-extension, and therefore not only a small-scale apparatus suffices, but also both process can be performed without removing the optical fibers, and the time required for manufacturing can be reduced to a great extent, and therefore the present invention can provide a wide-band optical fiber coupler at an economical price.
Also, to solve the above-mentioned problem, the present invention has developed a method of manufacturing an optical fiber coupler which consists of a process of arranging in parallel coat-removed parts of a plurality of optical fibers wherein coating of the intermediate part is removed by using arranging jigs disposed in an opposite fashion, a process of extending the above-mentioned plurality of optical fibers by a predetermined length using a low-temperature heat source softening the fibers disposed between the arranging jigs, a process of closely touching the extended parts by rotating at least one of the arranging jigs by an angle at which the parts having the smallest diameter of a plurality of optical fibers contact one another, a process of fusion-bonding the above-mentioned extended parts closely touched by a high-temperature heat formed by raising the temperature of the above-mentioned low-temperature heat source, and a process of lowering the temperature of the above-mentioned high-temperature heat source to form a low-temperature heat source again and extending the above-mentioned fusion-bonded part until a desired coupling is obtained.
The use of the manufacturing method of the present invention not only stabilizes the conditions of fusion-bonding of optical fibers in fusion-bonding, but also extremely improves the reproducibility of the optical characteristic of the fabricated fusion-bond-extension type optical fiber coupler because of fabricating the coupling region without applying twist in fusion-bonding and extending. Particularly, this method is very effective in fabricating a wide-band optical coupler reducing the dependency on wavelength of the coupling ratio of worse reproducibility.
Furthermore, to solve the above-mentioned problem, the present invention has been achieved in a manner that in a manufacturing apparatus of an optical fiber coupler wherein a coupling region is formed by fusion-bonding and extending parts of a plurality of optical fibers arranged in a closely touched fashion, driving beds are installed which independently hold at least one optical fiber among a plurality of the above-mentioned optical fibers and the other optical fibers, and can extend the above-mentioned held optical fibers at speeds different from each other.
The use of the manufacturing apparatus of the present invention makes it possible to perform pre-extension and fusion-bond-extension, and therefore not only can the apparatus be made smaller-sized, but also each process can be performed continuously without removing the optical fibers, and therefore the apparatus of the present invention can reduce the time required for manufacturing, and makes it possible to provide a wide-band optical fiber coupler at an economical price.